Combustion apparatus with direct firing system

ABSTRACT

The invention relates to a combustion apparatus, in particular to a burner for the combustion of carbonaceous fuel. The combustion apparatus comprises a burner having a burner inlet for receiving a supply of combustible pulverous fuel and a supply of comburant gas and a burner outlet in the vicinity of which combustion of the fuel is supported during use; at least a primary conduit defining a flow channel for conveying a mixture of fuel and comburant gas from the burner inlet to the burner outlet; a fuel feed line defining a flow channel which conveys a pulverous combustible fuel in a dense phase; and a supply conduit fluidly connecting a comburant gas supply to the primary conduit and defining with the primary conduit a primary flow stream; wherein the fuel feed line is provided with a fuel feed outlet into the supply conduit upstream of the burner inlet to supply pulverous combustible fuel in a dense phase; and wherein the primary flow stream is provided with a mixing device downstream of the fluid supply outlet. A method for combusting pulverous combustible fuel in a burner embodying such principles is also described.

The present invention relates to a combustion apparatus, in particularto a burner for the combustion of carbonaceous fuel. In the preferredcase the invention relates to a burner for particulate solidcarbonaceous fuel having an indirect firing system and is for example anindirect fired pulverous coal fired burner. In particular, but notexclusively, the invention relates to a combustion apparatus capable ofair and oxyfuel firing and utilizing flue gas recirculation. For examplethe invention relates to a burner for use in a power generationapparatus and to a power generation apparatus including one or more suchburners.

In the field of power generation from fossil fuels, and in particularfrom coal, conventional plant generally operate a direct firing system,where raw coal is fed from a bunker to the mill and pulverized and thenthe pulverized coal is conveyed directly from the mill to the burner.However, with the development of technologies, such as coal pre drying,and particularly lignite pre drying, it is becoming more common thatplant operate an indirect firing system, where the raw coal is milled toa bunker and then the dried pulverized coal fed from the bunker to theburner. This allows for conveying of the coal at a higher solid to gasmass ratio than conventionally used in direct fired systems.Conventionally and hereafter this is referred to as “dense phase”conveying of the pulverized fuel.

There is significant prior experience relating to the efficientcombustion of solid fuels for steam raising applications. There has beena strong drive in recent years to reduce levels of emissions of oxidesof nitrogen from carbonaceous fuel burners, especially in the field ofpower generation. Low NOx burners have been developed which reduce theformation of NOx combustion gases.

In general terms, a low NOx burner for coal firing may comprise a numberof components, which may include:

-   -   a pipe to supply the pulverized fuel and the conveying air        (often known as “primary” air)    -   a number of channels arranged concentrically around the        pulverized fuel supply, through which the burner combustion air        is supplied; in a low NOx burner there will typically be two or        more channels for the combustion air, these are often known as        “secondary” air, “tertiary” air, etc.    -   devices to induce a swirling motion in the combustion air may be        placed in the secondary and tertiary (etc.) channels, these        devices may (optionally) be adjustable    -   devices to stabilise the flame, often placed on the end of the        fuel supply pipe and sometimes known as the “flame-holder”    -   devices placed inside the fuel supply pipe to control the fuel        distribution at the outlet of that pipe    -   supplementary equipment, such as igniters, light-up burners,        flame monitoring sensors, etc., optionally installed in a        separate tube, which may be located centrally within the fuel        pipe where it is known as the “core” air tube; the core air tube        may have its own air supply; alternatively supplementary        equipment may be installed in other locations in the burner or        close by.

Where “air” is used herein the skilled person will readily appreciatethat other oxygen containing comburant gases may be substituted in thefamiliar way for example for oxyfuel firing including a comburant gashaving a reduced nitrogen content relative to air, for examplecomprising mixtures of pure oxygen and/or recycled flue gas and/or air.

FIG. 1 (prior art) shows one specific implementation of low NOx burnertechnology; it is known by those knowledgeable in this area that thereare a number of variant low NOx burner designs available.

Implicit in the design of any low NOx burner is the presumption thatthere must be sufficient oxidant in the primary (transport) stream tosupport the early combustion of the solid fuel; a natural progressionfrom the use of direct fired combustion systems is that the air flowrequired to convey the solid fuel from the pulverizing mill is alsosufficient for the early combustion of that solid fuel. It has beenshown that reducing the amount of air (oxidant) available in the earlystages of combustion has a detrimental impact upon the stability of theflame and, at a certain level, the flame will extinguish, even though asufficient quantity of air is supplied to the burner as a whole.

This has tended to discourage consideration of the use of indirectfiring for low NOx burners.

US2009/0000532 describes a low-NOx burner suitable for firing thefurnace of a steam generator which is equipped with a dense phaseconveyance of the fuel. However, the arrangement requires burnerredesign to provide for separate conveying of primary fuel and primaryoxidant into the burner via concentric primary air and fuel conduits andto provide for mixing of the fuel and primary oxidant streams within theburner downstream of the burner entrance and towards the burner outletby provision of swirl devices in the primary conduit. This leads to amore complex design of burner.

An arrangement which is simpler and/or more immediately compatible withexisting low-NOx burner designs is desirable.

According to the invention there is provided a combustion apparatuscomprising:

a burner having a burner inlet for receiving a supply of combustiblepulverous fuel and a supply of comburant gas and a burner outlet in thevicinity of which combustion of the fuel is supported during use;

at least a primary conduit defining a flow channel for conveying amixture of fuel and comburant gas from the burner inlet to the burneroutlet;

a fuel feed line defining a flow channel which conveys a pulverouscombustible fuel in a dense phase; and

a supply conduit fluidly connecting a comburant gas supply to theprimary conduit and defining with the primary conduit a primary flowstream;

wherein the fuel feed line is provided with a fuel feed outlet into thesupply conduit upstream of the burner inlet to supply pulverouscombustible fuel in a dense phase;

and wherein the primary flow stream is provided with a mixing devicedownstream of the fluid supply outlet and in particular at or in thevicinity of the burner inlet.

Fundamentally, the burner of the invention is an indirectly fired burnerin that the fuel feed line conveys pulverous fuel to the burner in adense phase for mixing with a supply of comburant gas at the burner,rather than a direct firing system. The invention offers a capability ofproviding a low-NOx burner, which is suitable for firing the furnace ofa steam generator, with a dense phase conveyance of fuel, withoutdisadvantageously affecting the low-NOx combustion characteristic of theburner.

However, it can be contrasted with US2009/0000532, where the burner hasseparate primary fuel and primary oxidant tubes and where mixing ofdense phase primary fuel and oxidant takes place within the burnertowards the outlet/combustion end by means of swirl devices specificallyprovided for such a purpose.

By contrast, in accordance with the present invention, mixing of thedense phase primary fuel and oxidant is effected upstream of the mainbody of the burner and in particular at or about the burner inlet andfor example immediately upstream of the burner inlet by means of asuitable flow stream mixing device. In particular this is done incombination with a duct arrangement upstream of the burner, whereby adense phase stream of solid pulverous fuel is introduced to a streamcontaining the oxidant (air or other oxygen containing gas) upstream ofthe mixing device, so as to create a mixture capable of sustaining astable flame in a solid fuel burner after these two streams are mixedwithin the burner following their interaction with a suitable mixingdevice. The mixing device is conveniently a static mixing device but mayadditionally or alternatively include a variable mixing device.

The resultant mixture capable of sustaining a stable flame is conveyedalong a primary conduit in essentially conventional manner to a burneroutlet, for example letting into a combustion chamber in familiarmanner, in the vicinity of which a combustion site is defined at whichcombustion of the fuel is supported during use.

The general design of the burner may thus otherwise be relativelyconventional. In particular, conventional low NOx burner designs may bereadily adapted in accordance with the principles of the invention. Thisis particularly advantageous in the case where the low NOx burner designto which the invention is to be applied already includes a suitablestatic or other mixing device (such as the scroll plate in the low NOxburner design given by way of example in FIG. 1) in the primary conduitat or about the burner inlet. In such cases, the existing mixing devicemay be used to complete the process of mixing the dense phase fuelstream and the primary comburant gas stream as it feeds into the primaryconduit. Conveniently therefore, a suitable mixing device such as asuitable static mixing device is provided within the primary conduit inthe vicinity of the burner inlet. Alternatively it is a relativelysimple design matter to provide a suitable mixing device in the primarystream at or about the burner inlet.

In all cases, the invention is distinctly characterised in that fuel issupplied in dense phase but that mixing of the fuel and primarycomburant gas stream is effected upstream of and/or in the vicinity ofthe burner inlet, rather than within the burner and/or towards theoutlet, and that the primary conduit within the burner carries in use afuel and oxidant mix capable of supporting combustion. This is effectedby the combination of injection of fuel in dense phase to the supplyconduit carrying oxidant upstream of the burner inlet and by provisionof a suitable mixing device such as a static mixing device downstreamthereof and for example at or about the burner inlet and for exampleimmediately upstream of the burner inlet.

In the context of the art of the invention, the term “dense phase” willbe readily understood by the person skilled in the art. It includes forexample conveying flows of pulverous fuel and transport gas withpulverous fuel to transport gas mass ratio of at least 3 and for exampleat least 5, at pressures of for example 0.5 to 5 bar, and at flow speedsof for example 10 to 30 ms⁻¹ or more. It should be emphasised that theseparameters are examples only. The skilled person will readily be able todetermine whether a particular conveying flow constitutes a dense phaseflow as it would be understood in the art.

In accordance with the invention a mixing device is provided in streamin the primary flow stream to facilitate mixing of the dense phasepulverous fuel stream and the primary comburant gas stream upstream ofthe burner or in the vicinity of the burner inlet. Conveniently themixing device comprises a static mixing device.

A suitable static mixing device comprises a static formation located inthe primary flow stream, and for example in the primary conduit at ortowards a burner inlet end thereof, configured to effect at least apartial obstruction of flow of the primary flow stream. In a preferredcase a static mixing device comprises a static formation configured toimpart a swirling motion to the primary flow stream.

For example a static mixing device may comprise one or more bladedformations presenting a flow deflection surface at an angle to a primaryflow direction of the stream, for example at an angle to an axial flowdirection. A static mixing device may comprise one or more helicalbladed formations. A static mixing device may comprise a scroll platesuch as is familiar for example from the prior art illustration of FIG.1.

Additionally, the supply conduit may be fluidly linked to the primaryconduit at an angle thereto, for example such there is an angle betweenan axial flow direction in the supply conduit and an axial flowdirection in the primary conduit. This produces a deviation in flowdirection in the primary flow stream downstream of the fuel outlet andat the burner inlet which may therefore assist in mixing of the densephase pulverous fuel stream and the primary comburant gas stream. Amixing device such as a static mixing device is conveniently located ator about this deviation in flow direction.

In the preferred case, the fuel feed line is provided with a fuel feedoutlet into the supply conduit closely upstream of the mixing device.The mixing device may be provided closely upstream of the burner inletand/or within the primary conduit of the burner in the vicinity of theinlet such that mixing is effected before or about the burner inletregion and such that for at least a substantially major part of theburner length the primary conduit carries in use a mixed fuel andprimary comburant gas supply.

The temperature of the mixture is below the devolatilisation initiationtemperature of the solid fuel.

It is a particular advantage although not a requirement of the inventionthat it can be applied to low-NOx pulverized coal burners followinggeneral known design principles without major additional modification.

In a preferred case, the burner of the invention is a pulverized coalburner. The invention is suitable for pulverous fuel burners forpulverized bituminous coals and for dried pulverized lower rank coalssuch as brown coals/lignites. The invention is particularly suited toapplication for pulverous fuel burners for dried pulverized lignites.

In a preferred case, the burner of the invention is a low-NOx burner.Known principles of low-NOx burner design may be embodied.

For example the burner may comprise one or more secondary and/or one ormore tertiary or higher order conduits comprising flow channels for thesupply of further gases such as further comburant gases to thecombustion site at the burner outlet. For example, as will be familiar,one or more secondary and/or one or more tertiary or higher orderconduits may be disposed annularly in concentric manner about a primaryconduit.

For example the burner may comprise a core tube about which the primaryconduit is annularly disposed. The core conduit may comprise a flowchannel for the supply of further gases such as further comburant gasesto the combustion site at the burner outlet and/or may include anignition lance for example coaxially arranged in the core tube.

Preferably, the combustion apparatus is adapted for oxyfuel firing. Thatis, the combustion apparatus comprises a comburant gas supply apparatusadapted to supply a comburant gas to at least one conduit of the burnerhaving a reduced nitrogen content relative to air. Preferably thecomburant gas supply apparatus is adapted to supply a comburant gas toat least one conduit of the burner that does not contain air. Preferablythe comburant gas supply apparatus is adapted to supply a comburant gasto at least one conduit of the burner that is substantially free ofnitrogen.

Optionally, the comburant gas supply apparatus may include a source ofpure oxygen, such that the comburant gas supply apparatus is adapted tosupply an oxidant mixture comprising pure oxygen and other gases to atleast one conduit of the burner.

Optionally, the comburant gas supply apparatus may additionally beadapted to supply comburant air. The comburant gas supply apparatus maybe adapted to switchably supply either air or the comburant gas having areduced nitrogen content relative to air to at least one conduit of theburner.

Preferably the combustion apparatus includes a flue gas recirculationconduit. Preferably the flue gas recirculation conduit is fluidlyconnected in series or parallel to the comburant gas supply apparatussuch that a comburant gas mixture including recycled flue gas may besupplied to at least one conduit of the burner.

Such a comburant gas supply apparatus facilitates control of thefuel/oxidant stoichiometry within the burner.

The parameters that are understood to affect the ignition and stabilityof the flame include, for example, the following;

-   -   the primary and secondary stream velocities    -   secondary/primary velocity ratio    -   the ignition zone stoichiometry    -   the ignition zone temperature/heat availability    -   the burner geometry, e.g. ratio of core air to primary air tubes        or ratio of primary air to secondary air tubes

Overall stoichiometry, the ratio of the total oxygen supplied forcombustion divided by the theoretical total oxygen required for completecombustion, and burner zone stoichiometry, the ratio of the total oxygensupplied to all (e.g. core, primary, secondary and tertiary) the burnerstreams for combustion divided by the theoretical total oxygen requiredfor complete combustion, are common stoichiometry parameters assigned tocontrol the performance of the burner in terms of combustion efficiencyand NOx production.

However, in relation to ignition of the flame it is possible to deriveto other stoichiometry parameters. One such parameter is the ignitionzone stoichiometry, the ratio of the total oxygen supplied to the coreand primary burner streams for combustion divided by the theoreticaltotal oxygen required for complete combustion. A second parameter is theignition zone volatile matter stoichiometry, the ratio of the totaloxygen supplied to the core and primary burner streams for combustiondivided by the theoretical total oxygen required for complete combustionof the volatile matter in the coal.

If the oxidant is air, then the stoichiometry parameters are directlyrelated to the mass, or volume, flow of air since the concentration ofoxygen in air is constant at 20.95% v/v or 23.14% w/w.

However, if the oxidant is a mixture of an inert gas (that could benitrogen, carbon dioxide, moisture, argon, etc., or a mixture of thesein, for example, flue gas) and oxygen, then the stoichiometry parametersare related to both the mass, or volume, flow of oxidant and theconcentration of oxygen in oxidant. For this application theconcentration of oxygen in oxidant is preferably in the range 10% v/v to35% v/v.

It has been found that mixing the dense phase solid fuel streamcontaining insufficient oxidant to support combustion by itself, with aprimary air (oxidant) stream to achieve the desired ignition zonestoichiometry and concentration of oxygen in oxidant allows the safe,stable, and efficient combustion of a solid fuel stream that is suppliedvia a dense phase conveying system. Significantly the arrangement testedallowed the primary air (oxidant) and solid fuel stream to replicate thetypical conditions exiting the pulverizing mills and commonly applied toburners of this type.

It has been found that relatively small instantaneous variations in thefuel flow to a burner have a substantial impact upon the residual oxygenlevel after combustion (example: for a typical power station burner with˜40 MWt thermal input firing coal the coal flow is around 5 tonne/h; aninstantaneous increase in the coal flow of just ˜200 g reduces the exitoxygen level from 3% v/v to zero, with the localised impact beingconsiderably greater in the early combustion region). Such variationscan have a destabilising effect on the flame as the early combustion istemporarily starved of oxygen. It is therefore shown to be importantthat, in a dense phase combustion system, the amount of primary air(oxidant) supplied to the burner is sufficient to accommodate the effectof the fluctuations in solid fuel feed rate that arise from the densephase feeding system.

It is well known by those familiar with the operation of pulverizedsolid fuel combustion systems that there is a risk of the fuel burningin an uncontrolled manner within the equipment should the temperature betoo high. To ensure that such uncontrolled burning does not occur, it isnecessary to maintain temperatures that are lower than the“devolatilisation initiation temperature”, that is the temperature atwhich the volatile material in the coal starts to be released. Thisdevolatilisation initiation temperature is dependant upon the solidfuel; for lignite coals it is typically around 250° C., for bituminouscoals it is ranges between ˜300° C. to ˜380° C., for low volatile &anthracitic coals it is around ˜400° C. to ˜520° C., and for certaintypes of petroleum coke it is around ˜350° C. to ˜380° C. Thus, inaddition to ensuring that a desirable ignition zone stoichiometry isachieved, the mixing of the dense phase stream and the primary air(oxidant) stream must be such that the mixture temperature is lower thanthe devolatilisation initiation temperature by a reasonable margin.

In accordance with the invention in a further aspect a method forcombusting pulverous combustible fuel in a burner having at least aprimary conduit defining a flow channel for conveying a mixture of fueland comburant gas from a burner inlet to a burner outlet comprises:

feeding comburant gas to the primary conduit via a supply conduitfluidly continuous therewith;

feeding the fuel to the supply conduit by a dense phase conveyance, viaa fuel supply outlet within the supply conduit in particular to a pointupstream and for example closely upstream of the burner inlet;

disposing a mixing device downstream of the fluid supply outlet and forexample at or in the vicinity of the burner inlet and in particularimmediately upstream of the burner inlet to effect mixing of thepulverous combustible fuel and comburant gas.

In accordance with the method of this aspect of the invention the burneris an indirectly fired burner. Mixing of the dense phase primary fueland oxidant is effected upstream of the main body of the burner and inparticular at or about the burner inlet, by supplying a dense phasestream of solid pulverous fuel into a stream containing the oxidant andby interaction with a suitable flow stream mixing device. The mixingdevice is conveniently a static mixing device but may additionally oralternatively include a variable mixing device. Thus, fuel is suppliedin dense phase but mixing of the fuel and primary oxidant is effectedupstream of and/or in the vicinity of the burner inlet, rather thanwithin the burner towards the outlet.

In particular, the mixing device, for example the static mixing device,is used to effect at least a partial obstruction of flow of the primaryflow stream of pulverous fuel in dense phase and comburant gas tofacilitate mixing thereof. In a preferred case the mixing device is usedto impart a swirling motion to the primary flow stream of pulverous fuelin dense phase and comburant gas to facilitate mixing thereof.Additionally, the primary flow stream may be caused to deviate in flowdirection at the mixing device and/or at the burner inlet to assist inmixing of the dense phase pulverous fuel stream and the primarycomburant gas stream.

Preferably, the pulverous fuel burner is a pulverized coal burner, forexample a burner for pulverized bituminous coal or dried pulverizedlower rank coal. Consequently preferably the pulverous fuel ispulverized coal, for example pulverized bituminous coal or driedpulverized lower rank coal. Preferably, the pulverous fuel is driedpulverized lignite.

Preferably, the burner is operated in low-NOx operation, in particularin that the supply of comburant gas is split between supply of comburantgas mixed with fuel to a primary conduit and supply of comburant gas toone or more secondary and/or one or more tertiary or higher orderconduits.

The method is suitable for air and/or oxyfuel firing and is particularlysuitable for oxyfuel firing. Preferably the comburant gas is one or moreof: air; oxygen; a comburant gas having a reduced nitrogen contentrelative to air; recycled flue gas. Suitable mixtures of the foregoingmay be used to control the fuel/oxidant stoichiometry within the burner.

In the particular case, the method of this aspect of the invention is amethod of operation of a burner of the first aspect of the invention andpreferred features of the method will be understood by analogy.

An embodiment of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is a sectional side view of a prior art burner to which theprinciples according to the invention could be applied;

FIG. 2 is a simplified diagrammatic view of a combustion apparatusaccording to the invention such as it might be applied for example to aburner of the type illustrated in FIG. 1.

FIG. 1 is a sectional side view of a prior art low-NOx burner 52 fittedto a boiler wall 12.

Each burner 52 has five co-axially arranged tubular partitions definingannular passages for one, or a mixture, of fuel, oxygen, flue gas, andair.

Each burner 52 has a primary tube 90 which is fluidly connected to theprimary fuel input conduit 42. In the prior art the burner 52 isdescribed for direct firing. The burner is adapted for air or oxyfuelfiring. A mixture of pulverized coal fuel and comburant gas, for exampleair for air firing, or a mixture of recycled flue gas and oxygen foroxyfuel firing, is supplied to a scroll plate 94 of the primary tube 90via a tangential connection 92.

Each burner 52 also has a secondary tube 100 which is fluidly connectedto the secondary input conduit 50. Comburant gas, for example air forair firing, or a mixture of recycled flue gas and oxygen for oxyfuelfiring, is supplied to apertures 102 provided in the secondary tube 100from a wind box 40 surrounding the burner 52.

Each burner 52 includes two tertiary tubes 106, 108. Also, a core tube110 is provided, which includes a radial connection 112. These tubes maybe fluidly connected to one or both of the combustion gas supply meansand the flue gas recirculation system, which is not shown on FIG. 1.

In operation, fuel within the primary tube 90 is given an axial and acircumferential momentum by the scroll plate 94. The scroll plate isintended to impart this axial and circumferential momentum to fuel in afuel gas mix which has already been mixed prior to transport to theburner.

The flow is discharged past a lip 114 as a vigorously eddying flow whichignites at the lip 114 defining an initial combustion region. Reducingconditions prevail within this region such that there is minimaloxidation of the nitrogen in the fuel. The amount of oxygen in the coretube 110 is also limited to maintain these conditions.

Flow from the secondary 100 and tertiary 106, 108 tubes forms anenvelope around the initial combustion region so that combustion of thefuel is completed downstream under oxidising conditions.

Such a burner and the general principles it embodies for low-NOxcombustion, will be familiar.

FIG. 2 shows how the principles of the invention might be applied to aburner of the type illustrated in FIG. 1.

A supply tube 1 carrying a concentrated stream of pulverous solid fueland a small amount of transport gas in dense phase is introduced into alarger tube 2 carrying the primary oxidant to the burner. FIG. 2 shows atypical arrangement for the dense phase solid fuel injection into theprimary stream and its subsequent mixing at the burner entry.

In a typical “dense phase” suitable for application of the principles ofthe invention, the transport gas stream could be air, inert gas, or agas mixture containing up to 21% v/v oxygen, including flue gas.Typically the mass ratio of solid fuel to gas would be of the order of5:1.

A suitable primary oxidant stream for application of the principles ofthe invention could be air or a gas mixture, including flue gas,containing oxygen, for example air for air firing, or a mixture ofrecycled flue gas and oxygen for oxyfuel firing. The concentration ofoxygen in the primary stream is preferably in the range 10 to 35% v/v.

Because mixing between two parallel streams is poor, especially whentheir velocities are similar, a suitable mixing device and preferably astatic mixing device is introduced. Conveniently in this embodiment thescroll casting 21, shown at the solid fuel and primary oxidant entry tothe burner 22, serves as a static mixing device. The scroll 21 providesan obstruction to the inlet flow, and imparts a swirling motion. In thisembodiment there is also a 90° change in direction between an axial flowdirection for the primary supply flow 2 and an axial flow direction forthe primary flow within the burner 2 a which further enhances the mixingof the solid fuel and primary oxidant.

Advantageously therefore in this embodiment a static mixing device isemployed which was already present in the direct fired design. However,the invention encompasses additionally or alternatively the provision ofspecific to purpose static and/or other mixing devices in the primarystream upstream of or at or about the inlet to the burner.

At the outlet of the scroll 21 the solid fuel and oxidant are well mixedin a controlled fashion, giving the desired solid fuel concentrationdistribution to promote stable combustion. The mixture forms a primaryflow 2 a through a primary tube to a burner outlet 30 in a furnace wall31 in familiar manner.

It is a particular advantage of the invention that it can be applied tolow-NOx pulverized coal burners following general known designprinciples without major additional modification. Thus, the remainder ofthe burner, as illustrated schematically, may include in any suitablecombination those features of the prior art burner illustrated in FIG.1.

For example the illustrated burner comprises secondary and tertiaryconduits 4 and 5 for introduction of supplies of secondary and tertiaryoxidant gas, for example air for air firing, or a mixture of recycledflue gas and oxygen for oxyfuel firing.

For example the illustrated burner comprises a core tube 3 about whichthe primary conduit is annularly disposed. The core tube may be used tosupply further oxidant gases or may include an ignition lance forexample coaxially arranged in the core tube.

The ignition zone stoichiometry at the outlet of a static mixing devicein an embodiment of the invention would be preferably in the range 0.1to 0.3 and for example about 0.2.

The benefit of the invention is that, by the use of a combination of adense phase fuel inlet pipe and a suitable flow mixing device in theprimary flow and in particular a suitable static mixing device, theprocess conditions (i.e. the solid concentration and ignition zonestoichiometry) may be modified to create conditions that favour theignition of the solid fuel and thereby enhance the flame stability inthe burner; this characteristic leads to improved operationalflexibility of the low NOx burner by allowing it to be used over a widerange of load, and ensuring that flame stability is robust with respectto the instantaneous variations in solid fuel feed rate as observed inpractical combustion systems. Additionally the mixture temperature ismaintained at a level lower than the devolatilisation initiationtemperature to ensure that uncontrolled burning does not occur withinthe combustion equipment hardware.

In accordance with the invention a solid fuel entry located immediatelyupstream of a mixer is thus used to deliver a controlled mixture ofsolid fuel and primary oxidant containing sufficient oxygen to sustaincombustion (including when the fuel flow is subject to fluctuation), andat a temperature lower than the devolatilisation initiation temperature.

1. A combustion apparatus comprising: a burner having a burner inlet forreceiving a supply of combustible pulverous fuel and a supply ofcomburant gas and a burner outlet in the vicinity of which combustion ofthe fuel is supported during use; at least a primary conduit defining aflow channel for conveying a mixture of fuel and comburant gas from theburner inlet to the burner outlet; a fuel feed line defining a flowchannel which conveys a pulverous combustible fuel in a dense phase; anda supply conduit fluidly connecting a comburant gas supply to theprimary conduit and defining with the primary conduit a primary flowstream; wherein the fuel feed line is provided with a fuel feed outletinto the supply conduit upstream of the burner inlet to supply pulverouscombustible fuel in a dense phase; and wherein the primary flow streamis provided with a mixing device downstream of the fluid supply outlet.2. A combustion apparatus in accordance with claim 1 wherein the primaryflow stream is provided with a mixing device at or about the burnerinlet.
 3. A combustion apparatus in accordance with claim 1 furtheradapted to effect mixing of the dense phase pulverous combustible fueland the comburant gas upstream of the burner inlet by means of a ductarrangement upstream of the burner, whereby a dense phase stream ofsolid pulverous fuel is introduced to a primary stream containing thecomburant gas.
 4. A combustion apparatus in accordance with claim 1wherein the mixing device comprises a static mixing device.
 5. Acombustion apparatus in accordance with claim 4 wherein the staticmixing device comprises a static formation located in the primary flowstream and configured to effect at least a partial obstruction of flowof the primary flow stream.
 6. A combustion apparatus in accordance withclaim 4 wherein the static mixing device comprises a static formationlocated in the primary conduit at or towards a burner inlet end thereof7. A combustion apparatus in accordance with claim 4 wherein the staticmixing device comprises a static formation configured to impart aswirling motion to the primary flow stream.
 8. A combustion apparatus inaccordance with claim 4 wherein the static mixing device comprises oneor more bladed formations presenting a flow deflection surface at anangle to a primary flow direction of the stream, for example at an angleto an axial flow direction.
 9. A combustion apparatus in accordance withclaim 4 wherein the static mixing device comprises one or more helicalbladed formations.
 10. A combustion apparatus in accordance with claim 4wherein the static mixing device comprises a scroll plate.
 11. Acombustion apparatus in accordance with claim 1 wherein the fuel feedline is adapted to convey a pulverous combustible fuel in a dense phasewith pulverous fuel to transport gas mass ratio of at least 3 atpressures of 0.5 to 5 bar and at flow speeds of at least 10 ms-1.
 12. Acombustion apparatus in accordance with claim 1 wherein the supplyconduit is fluidly linked to the primary conduit at an angle thereto,such that there is an angle between an axial flow direction in thesupply conduit and an axial flow direction in the primary conduit.
 13. Acombustion apparatus in accordance with claim 12 wherein a mixing deviceis located at or about the deviation in flow direction.
 14. A combustionapparatus in accordance with claim 1 wherein the burner is a pulverizedcoal burner.
 15. A combustion apparatus in accordance with claim 1wherein the burner is a low-NOx burner.
 16. A combustion apparatus inaccordance with claim 1 wherein the burner comprises one or moresecondary and/or one or more tertiary or higher order conduitscomprising flow channels for the supply of further gases such as furthercomburant gases to the combustion site at the burner outlet.
 17. Acombustion apparatus in accordance with claim 16 wherein one or moresecondary and/or one or more tertiary or higher order conduits aredisposed annularly in concentric manner about a primary conduit.
 18. Acombustion apparatus in accordance with claim 1 wherein the burnercomprises a core tube about which the primary conduit is annularlydisposed.
 19. A combustion apparatus in accordance with claim 1 adaptedfor oxyfuel firing.
 20. A combustion apparatus in accordance with claim19 wherein the comburant gas supply apparatus includes a source ofcomburant gas having a reduced nitrogen content relative to air.
 21. Acombustion apparatus in accordance with claim 20 wherein the comburantgas supply apparatus is additionally adapted to supply comburant air.22. A combustion apparatus in accordance with claim 21 wherein thecomburant gas supply apparatus is adapted to switchably supply eitherair or the comburant gas having a reduced nitrogen content relative toair to at least one conduit of the burner.
 23. A combustion apparatus inaccordance with claim 1, further comprising a flue gas recirculationconduit fluidly connected in series or parallel to the comburant gassupply apparatus such that a comburant gas mixture including recycledflue gas may be supplied to at least one conduit of the burner.
 24. Amethod for combusting pulverous combustible fuel in a burner having atleast a primary conduit defining a flow channel for conveying a mixtureof fuel and comburant gas from a burner inlet to a burner outletcomprising the steps of: feeding comburant gas to the primary conduitvia a supply conduit fluidly continuous therewith; feeding the fuel tothe supply conduit by a dense phase conveyance, via a fuel supply outletwithin the supply conduit in particular to a point closely upstream ofthe burner inlet; disposing a mixing device downstream of the fluidsupply outlet to effect mixing of the pulverous combustible fuel andcomburant gas.
 25. A method in accordance with claim 24 comprising thestep of disposing a mixing device at or about the burner inlet.
 26. Amethod in accordance with claim 24, wherein mixing of the pulverouscombustible fuel and comburant gas is effected by supplying a densephase stream of solid pulverous fuel into a stream containing theoxidant and by causing subsequent interaction of the flow of combustiblefuel and comburant gas is with a suitable flow stream mixing device. 27.A method in accordance with claim 24, wherein the mixing device is astatic mixing device.
 28. A method in accordance with claim 24, whereinthe mixing device is used to effect at least a partial obstruction offlow of the primary flow stream of pulverous fuel in dense phase andcomburant gas to facilitate mixing thereof.
 29. A method in accordancewith claim 24, wherein the mixing device is used to impart a swirlingmotion to the primary flow stream of pulverous fuel in dense phase andcomburant gas to facilitate mixing thereof.
 30. A method in accordancewith claim 29, wherein the primary flow stream is caused to deviate inflow direction at the mixing device and/or at the burner inlet to assistin mixing of the dense phase pulverous fuel stream and the primarycomburant gas stream.
 31. A method in accordance with claim 24, whereinthe burner is operated in low-NOx operation.
 32. A method in accordancewith claim 24, wherein the comburant gas is one or more of: air; oxygen;a comburant gas having a reduced nitrogen content relative to air;recycled flue gas.